Low cost brazes for titanium

ABSTRACT

A braze material and method of brazing titanium metals. The material may consist of Ti, Ni, Cu Zr, PM and M where PM is a precious metal and M may be Fe, V, Cr, Co, Mo, Nb, Mn, Si, Sn, Al, B, Gd, Ge or combinations thereof, with the (Cu+PM)/Ni ratio around 0.9. Optionally, a second brazing may be performed to rebraze any braze joint that did not braze successfully. The second brazing material has a lower braze temperature than the first and may consist of a mixture of Ti, Ni, Cu, Zr PM and M with from 1-20 wt % more Zr, PM, M or combinations thereof than the first braze. The braze material may be placed on a base material, in a vacuum furnace, and heated to form a braze joint between the braze and base material. The heating step may occur from about 800-975° C. and over 3 to 15 minutes.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 10/621,071, filed Jul. 14, 2003.

BACKGROUND OF THE INVENTION

The present invention generally relates to a material for and method ofjoining metals and, more particularly, to improved materials and methodsfor brazing titanium alloys including Beta 21S.

Titanium alloys have been of considerable interest in many applicationsdue to their desirable performance characteristics. Such alloys providelow density, high strength, fatigue resistance, corrosion resistance,and good strength-to-weight ratio. Because titanium alloys weigh a greatdeal less than stainless steel and nickel based alloys, they haveprovided great advances in many fields where there is a constant driveto minimize design weight. For instance in aerospace applications,aircraft heat exchangers and the like greatly benefit from reducedweight. The operating conditions of heat exchangers also involve highstresses induced by pressure and temperature, combined with fatigueloading. Another desirable characteristic of titanium alloys is theirability to withstand high temperatures, as the temperature in aircraftheat exchangers can be in excess of 1000° F.

Of the various titanium alloys that exist, metastable beta-titaniumalloys are of great interest, particularly in aerospace applicationsrequiring highly formable sheet metal or foil gages. One of the mostpromising beta alloys is Beta 21S, i.e., a beta alloy containing about21% of alloying additions. Beta 21S, Ti-15Mo-3Nb-3Al-0.2Si, wasdeveloped to overcome some of the disadvantages of the other titaniumalloys. As an example, alpha-beta alloys tend to have poor formability,while other beta alloys tend to have reduced elevated temperatureproperties. In contrast, Beta 21S has good formability, good elevatedtemperature properties, low density, and oxidation resistance.

Yet, the ability to employ Beta 21S in aerospace or other applicationsis limited by the ability to join pieces of Beta 21S together. Withoutthe ability to adequately join, any application is limited in size andcomplexity. That is, the application is limited by the ability to makeand form a single piece of a base material large enough to make thefinal product. If a mechanical joining process is needed to joinmultiple pieces of base material, weight savings from the base materialitself may be lost. And the product design may require changes toaccommodate a mechanical fastener. Additionally, the ability to attachobjects to the base material can become limited by the physical presenceof a mechanical fastener which might need to be located at the joinerpoint between the base material and the object.

On the other hand, the advantages of a non-mechanical joining process ofbase materials can be significant. Some non-mechanical or metallurgicaljoining processes have included welding, diffusion bonding and brazing.The advantages of non-mechanical joining can be most evidentparticularly when the base material is of a thin gage type and, thus,weight savings are increased. A thin gage material might be of an orderaround 0.002 to 0.090 inches thick. Also, and unlike a mechanicalfastener, a non-mechanical joint can minimize the disadvantages ofjoining an object where the base material is itself joined. Thisminimization is achieved since the bulk or space occupied by mechanicalfasteners are omitted.

Still, there are disadvantages from non-mechanical joining. Thedisadvantages can include excessive alloying, metallurgicalinteractions, dissolution and erosion of base materials, and degradationof mechanical properties. In spite of the disadvantages and because ofthe advantages provided by titanium alloys, including Beta 21S,considerable effort has been made in the past to improve non-mechanicaljoining. Much of the effort has recently focused on brazing.

Brazing may be generally characterized as exposing the base material andbraze material to a temperature sufficient to cause the braze materialto melt. The atoms from the braze material then interdiffuse with theatoms in the base material. Upon the braze material solidifying, a jointis formed. While the general brazing process appears to be clear andstraightforward in principle, research indicates to the contrary. Thequality of the braze joint is highly dependent upon various factors inthe brazing process, such as temperature, rate of heating and cooling,composition of the braze material and composition of the base material.While the attempts to determine the effect of these factors have beenmany, their interdependent relationships remain less clear.

As an example of temperature and braze material dependency, C. Cadden etal., “Microstructural Evolution and Mechanical Properties and BrazeJoints in Ti-13.4 Al-21.2 Nb,” Welding Research Supplement, pp. 316-325s(August 1997) addressed an alpha-two Ti base material with a Ti—Cu—Nibraze material. Cadden et al. indicate that, depending upon the brazetemperature, the braze joint can have a room temperature tensilestrength comparable to alpha-two and an elevated temperature (649° C.and 760° C.) tensile strength of 70 to 80% of the base metal tensilestrength. Even with different nickel contents in the braze material, theaverage nickel content in the joint was found to be nearly constant.However, as between a rapidly solidified melt-spun braze foil and alaminated braze foil, the latter produced higher levels of nickel in thecenterline of the joint, which was believed to lead to poorer roomtemperature tensile behavior. This indicates that the homogeneousrapidly solidified braze may have advantages over the braze comprised offoils (Cu/Ti/Ni) laminated together. However, melt spinning is anexpensive process and cannot produce foil. Rather, melt spinning onlyproduces narrow strips of varying thickness, which are difficult to use.Planner Flow Casting was developed to produce braze foil with a rapidsolidified or amorphous structure. However, for Ti and Zr alloys, thenozzles used to cast the liquid metal and control its dimensions arequickly eroded and economic production of the desired braze foils is notpossible.

In another study of how temperature can affect the braze joint, T.Onzawa et al., “Brazing of Titanium Using Low-Melting Point Ti-BasedFiller Metals,” Welding Research Supplement, pp. 462-467s (December1990) investigated the base materials of commercially pure titanium(CPTi) and Ti-6 Al-4 V. The different filler metals used with the basematerials included Ti-37.5Zr-15Cu-10Ni, Ti-35Zr-15Cu-15Ni andTi-25Zr-50Cu. Onzawa et al. concluded that brazing above the alpha-betatransformation temperature and the beta transus temperature of the basemetal would cause the grains in the base metal to coarsen and fineWidmanstatten structure to form at the joint area. This resulted in poormechanical properties. Below the transition temperatures, the finegrains of the base metals were preserved, as well as the braze zonebeing distinct from the braze metal. This led to better mechanicalproperties. Onzawa et al. also determined that a shorter holding time ata braze temperature could improve the mechanical properties.

The effect of temperature and cooling rate on Beta 21S was described byHuang et al., “Effect of Heat Treatment on the Microstructure of aMetastable .beta.-Titanium Alloy,” Journal of Materials Engineering andPerformance, v. 3(4), pp. 560-566 (August 1994). They found that alphaprecipitated preferentially on the grain boundaries during highertemperature aging and within the grains during lower temperature aging.High temperature solutioning produced a coarse grain size, whileresolutioning treatment followed by slow cooling (such as duringbrazing) resulted in alpha precipitation. But with air cooling,precipitation was suppressed.

Another temperature and cooling rate investigation involved Ti—Pd andTi-6Al-4 V alloys brazed with 25Ti-25Zr-50Cu. Botstein et al. “Brazingof titanium-based alloys with amorphous 25 wt. % Ti-25 wt. % Zr-50 wt. %Cu filler metal,” Materials Science and Engineering, pp. 305-315 (1994).Botstein et al. determined that high heating and high cooling ratecreated only traces of Widmanstatten structure at the joint interface.On the other hand, low heating and low cooling rate tended to result ina coarse dendritic structure having high microhardness and low fracturestrength.

In investigating the confluence of temperature, cooling rate, brazematerial and base material, Rabinkin, “New Applications for RapidlySolidified Brazing Foils,” Welding Journal, pp. 39-46 (October 1989)described rapid solidification as a process having high cooling ratesthat allow stabilization of alloys into an amorphous state. Because suchalloys provide “instant melting,” Rabinkin indicated that they can beused to braze at lower temperatures and for a shorter time. As pointedout, these features are well suited to brazing items such as fine-gaugehoneycomb which require protection from molten filler metals. Moreimportantly, according to Rabinkin, is the ductility of the alloys, thuseliminating the need for large joint clearances to fill the brazecross-section. Rapidly solidified foils of 75Ti-15Cu-15Ni (sic) and83.1Zr-16.9Ni were considered by Rabinkin to be advantageous fortitanium base materials. On the other hand, Rabinkin indicated thatbraze powders have drawbacks that the rapidly solidified foils overcome.Apparently included in the group of disadvantageous powders isTi—Zr—Cu—Ni which had been used on titanium based alloys, includingtubing and honeycomb aircraft structures. However, we have alreadyindicated that the plannar flow casting technique used to produce theseamorphous foils relies upon a nozzle that is quickly eroded by molten Tiand Zr, resulting in short production times and high costs. There arealso problems associated with casting the Ti in air, as usual occurswith planar flow casting. Casting in vacuum or inert gas is possible,but significantly increases costs and makes the production of foil witha uniform thickness and width even more difficult. A variation in foilthickness will affect the ability of the foil to produce a good braze,as the gap between the two parts to be brazed varies. For example, insome areas the braze may not touch the two parts to be brazed, andtherefore may not braze them both. Also, variations in width require thefoil to be hand cut and positioned. This also results in increasedcosts.

U.S. Pat. No. 6,149,051, issued to (AlliedSignal) Honeywell discloses amethod of brazing a Ti-15 Mo-3 Nb-3 Al-0.2 Si base material includingthe steps of coating a braze material onto a base material. The brazematerial comprises substantially only a Ti—Cu—Ni—Zr mixture, with themixture comprising about 40% Ti, 20% Cu, 20% Ni and 20% Zr. The brazematerial is heated then a braze joint is formed between the braze andbase materials. The heating step can occur from 760 to 932° C. and over15 to 90 minutes. While this solves many of the problems of incompletebrazing, it is a very costly method especially because of the additionof the extremely expensive Zr, but also because the techniques to makethe braze alloy, such as powder atomization, mechanical alloying ormechanically commutation (i.e. breaking up a large ingot to powder), areexpensive. Also, the brazing must be closely controlled requiring aninitial heating and a subsequent heating. The '051 patent briefly statesthat as well as an alloyed powder the braze composition might beobtained by admixing elemental powders, but gives no details. This isbecause while this approach is recognized as feasible it has not beenconsidered desirable. Problems with it are that the braze is verynon-homogeneous and therefore melts over a wide temperature range makingbrazing difficult and producing a poor low strength braze joint. We havealready discussed the problems described by Rabinkin “New Applicationsfor Rapidly Solidified Brazing Foils,” Welding Journal, pp. 39-46(October 1989) for alloyed powders. These are significantly increasedfor elemental admixed powders, which would be even more“disadvantageous” than prealloyed powders. Another problem is that theseelemental powders have high oxide contents, which also reduces theproperties of the braze. It would be desirable to use these admixedelemental powders in order to reduce cost, but this would require thedisadvantages of a wide melting range, non-homogeneous brazemicrostructure and composition and the high oxide problem to be solved.

Further, it would be desirable to have a braze that allows for a secondbrazing at a lower temperature. For example, a heat exchanger may have1000 joints that need to be brazed. The first braze may only effectivelybraze 90% of the joints, leaving 10% or 100 joints with leaks. It wouldbe desirable to go back and rebraze the 100 joints that did not properlybraze, rather than scrap the part. However, not at the risk of harmingthe 90% of the brazes that properly brazed. The present invention solvesthis problem, by providing a first brazing and optionally a secondbrazing with a lower melting temperature, yet at the same time no orminimal amounts of Zr.

Further, it would be desirable to decrease the thermal exposureexperienced by the titanium parts being brazed. The cited prior worksdescribe rapid heating and cooling cycles and the short times attemperature. A reduction in the braze temperature, as we show to bepossible, would greatly reduce thermal exposure.

As can be seen, there is a need for improved methods for brazing betatitanium alloys. There is a particular need for improved brazing methodsthat are less temperature and/or time dependent such that processingparameters need not be so tightly controlled, and there is also arelated need to reduce braze temperature. A further need is a brazewhich is does not erode the titanium substrate. This is particularlyimportant for thin foil, as any erosion will quickly melt through thefoil. Erosion occurs due to the required use of braze temperatureshigher than the melt temperature of the braze material. Also, erosionoccurs because the melt temperature of the braze changes as Ti from thesubstrate is dissolved into the braze. This in turn changes itscomposition, i.e. the use of a non-optimum braze composition. There isalso a need to produce a homogeneous braze microstructure, without theexpense of rapid solidification techniques. Likewise, there is a need toprovide an improved brazing method, which enables other objects to bewelded at the brazed joint to further fabricate a complex assembly, butwithout deteriorating the brazed joint. There is a further need toprovide a brazing method which can utilize a powder braze material whichis easy and economical to formulate. Also, there is a need for a brazematerial that allows for a second brazing, when needed, at a lowertemperature.

SUMMARY OF THE INVENTION

In one aspect of the invention, a brazing material and method forjoining titanium metals is disclosed. This brazing material and itsmethod of use may be particularly well suited for aerospaceapplications, and more particularly for use within heat exchangers. Thebrazing material may have 44-70% by weight Ti; 15-24% by weight Ni, and15-20% Cu. Preferably, the Cu/Ni ratio is around 0.8. The method mayhave a first brazing step, which does not require Zr, which as discussedpreviously is very expensive. However, a second heating may beoptionally utilized which adds a minimal amount of Zr and/or a preciousmetal (PM) such as Ag, 1 to 12% to the first braze, to provide a highpercentage of brazes, yet contain the costs associated. (Ag is ˜⅓ thecost of Zr.) The use of the Zr and/or the PM addition lowers the meltingtemperature of the braze, thus providing a second melting point of thebraze material.

In one aspect of the invention, a brazing material and method forjoining titanium metals is disclosed. This brazing material and itsmethod of use may be particularly well suited for aerospaceapplications, and more particularly for use within heat exchangers. Thebrazing material may have 44-70% by weight Ti, 15-24% by weight Ni, and15-20% by weight Cu and Ag or other precious metal, PM. Preferably the(Cu+PM)/Ni ratio is around 0.9. The method may have a first brazingstep, which does not require Zr, which as discussed previously is veryexpensive. However, a second heating may be optionally utilized whichadds a minimal amount of either Zr or more Ag (PM) in the amount of 1 to12% by weight to the first braze, to provide a high percentage ofbrazes, yet contain the costs associated. The Zr or Ag (PM) lowers themelting temperature of the braze, thus providing a second melting pointof the braze material.

In another aspect of the present invention, a method of brazing atitanium metal is disclosed comprising the step of coating a brazematerial onto a base material. The braze material may be a mixture ofTi, Cu, Ni powders comprising 44-70 wt % Ti, 10-30 wt % Cu, 10-30% byweight Ni, with preferably a Cu/Ni ratio of around 0.8. It wassurprisingly found that when the particle size of the powders is lessthan 60 μm many of the previously discussed problems are alleviated. Forexample, problems associated with admixed elemental powders disappeared.Also, strong homogeneous braze joints could be formed with the brazemelting over a narrow braze temperature range. Thus, surprisinglyproducing results similar to a good prealloyed braze powder or even arapidly solidified braze. The base material with the braze material maybe placed in a vacuum furnace and heated for a given braze time, up to atemperature that is not more than a braze temperature of the brazematerial, to achieve thermal stability between the braze material andthe base material. This may result in a braze joint between the brazeand the base material.

In yet another aspect of the present invention, a method of brazing abeta phase titanium metal is disclosed comprising the step of coating afirst braze material onto a base material. The first braze material maybe a mixture of Ti, Cu, and Ni powders, 44-70 wt % Ti, 10-30 wt % Cu,10-30% by weight Ni, with preferably a Cu/Ni ratio of around 0.8. Thebase material with the braze material may be placed in a vacuum furnace,and a first heating performed to achieve thermal stability between thebraze material and the base material. The first heating is up to atemperature that is not more than a first braze temperature of the brazematerial. A second braze material may be coated onto the base material.The second braze material being a mixture of Ti, Cu, Ni, and Zr powderscomprising 44-70 wt % Ti, 10-30 wt % Cu, 10-30% by weight Ni, and0.5-12% by weight Zr, and a second heating of the braze materialperformed up to a second braze temperature, forming a braze jointbetween the braze and the base material.

In yet another aspect of the present invention, a method of brazing atitanium metal within a heat exchanger is disclosed comprising the stepof coating a braze material onto a multitude of portions of a basematerial. The braze material being a mixture of Ti, Cu, Ni powderscomprising 57 wt % Ti with a particle size of ≦20 μm, 19 wt % Cu with aparticle size of ≦20 μm, and 24% by weight Ni with a particle size of≦20 μm. The base material with the braze material coated, on a multitudeof portions, may be placed in a vacuum furnace and heated with a fiveminute hold at temperature to achieve thermal stability. The heating isup to a brazing temperature of 940° C. A multitude of braze joints areformed between the braze and the base material. The braze joints may beexamined to determine whether a second brazing is needed and/ordesirable. If needed and/or desirable, a second brazing may be performedcomprising the steps of coating a multitude of selected portions of thebase material with a second braze material. The second braze materialmay be obtained by simply adding a small quantity of Zr or Ag (PM)powder to the first braze forming a mixture of Ti, Cu, Ni and Zr powderscomprising 54 wt % Ti with a particle size of ≦20 μm, 18 wt % Cu with aparticle size of ≦20 μm, 23% by weight Ni with a particle size of ≦20μm, and 5% by weight Zr or Ag (PM) with a particle size of ≦20 μm. Thebase material with the second braze material may again be placed in avacuum furnace and heated for a given braze time to achieve thermalstability. The second braze material and the base material may be heatedto a temperature that is not more than a second brazing temperature ofsaid second braze material to form a braze joint between said braze andsaid base material. The second brazing temperature is around 900° C.,safely less than the first brazing temperature so the first braze jointsare not affected by the second brazing operation.

In yet another aspect of the present invention, a method of brazing anisomorphous beta phase only titanium base material to form a heatexchanger is disclosed comprising the step of coating a braze materialonto a multitude of portions of a base material. The braze material maybe a mixture of Ti, Cu, Ni, Ag (PM) powders comprising 50 wt % Ti with aparticle size of ≦20 μm, 18 wt % Cu with a particle size of ≦20 μm, 22%by weight Ni with a particle size of ≦20 μm, and 10% by weight Ag (PM)with a particle size of ≦20 μm. The base material with the brazematerial may be placed in a vacuum furnace. A first heating may beperformed on the braze material and the base material for five minutesto achieve thermal stability between the braze material and the basematerial. The heating may be between 910° C. and 920° C. A multitude ofbraze joints may be formed between the braze material and the basematerial. The braze joints may be examined to determine whether a secondbrazing is desirable. If a second braze is desired, a second brazing maybe optionally performed, comprising the steps of coating a multitude ofselected portions of the base material with a second braze material. Thesecond braze material may be a mixture of Ti, Cu, Ni, Ag (PM) and Zrpowders comprising 45 wt % Ti with a particle size of ≦20 μm, 20 wt % Cuwith a particle size of ≦20 μm, 20% by weight Ni with a particle size of≦20 μm; 10% by weight Ag (PM) with a particle size of ≦20 μm, and 3% byweight Zr with a particle size of ≦20 μm. The base material with thesecond braze material may be placed in a vacuum furnace and a secondheating performed. The second heating may be up to a second brazetemperature between 870° C. and 880° C. and at a rate in a range of 1.1to 7.7° C./minute. This may form a braze joint between the second brazematerial and the base material. The braze joint may be cooled andsolidified. In the unlikely case that all the braze joints are not nowgood then a third brazing operation may be performed by adding aslightly higher % of Zr and Ag (PM) to the first and second brazematerial. The third braze material may be a mixture of Ti, Cu, Ni, Ag(PM) and Zr powders comprising around 29 wt % Ti with a particle size of≦20 μm, 21 wt % Cu with a particle size of ≦20 μm, 20% by weight Ni witha particle size of ≦20 μm, 20% by weight Ag (PM) with a particle size of≦20 μm, and 15% by weight Zr with a particle size of ≦20 μm. The thirdheating may be performed in the range of 815 to 825° C. Again safelybelow the temperature used to form the successful brazes in the firstand second operation so that these are not affected, de-brazed, by thethird operation.

According to another aspect of the present invention, a method ofbrazing a titanium metal is disclosed comprising the steps of coating afirst braze material onto a base material, the first braze materialbeing based on the Ti—Ni—Cu, Ti—Ni—Cu—Ag, Ti—Ni—Cu—Zr or Ti—Ni—Cu—Ag—Zrsystems already described to which is added 1 to 20% of M, where M maybe Fe, V, Cr, Co, Mo, Nb, Mn, Si, Sn, Al, B, Gd, Ge or combinationsthere of. M is added to in order to further reduce the brazetemperature, substitute for the expensive elements such as Zr or toimprove desirable properties, such as corrosion and oxidation protectionby reducing for instance the Cu content. Ti, Cu, Ni and M 30-80 wt % Ti,10-30 wt % Cu, 10-30 wt % Ni and 1-20 wt % M; placing the base materialwith the braze material in a vacuum furnace; performing a first heatingof the braze material and the base material to achieve thermal stabilitybetween the braze material and the base material, the first heatingbeing up to a temperature that is not more than a first brazetemperature of the braze material; coating a second braze material ontothe base material, the second braze material being a Ti, Cu, Ni, M andZr or Ag (PM) comprising 30-70 wt % Ti, 10-30 wt % Cu, 10-30 wt % Ni,1-20 wt % M, and 0.5-15% by weight Zr and or Ag (PM); performing asecond heating of the braze material and the base material up to asecond braze temperature; and forming a braze joint between the brazeand the base material.

According to another aspect of the present invention, a method ofbrazing a titanium metal is disclosed comprising the steps of coating afirst braze material onto a base material, the first braze materialbeing based on the Ti—Ni—Cu, Ti—Ni—Cu—Ag, Ti—Ni—Cu—Zr or Ti—Ni—Cu—Ag—Zrsystems already described to which is added 1 to 20% of M with aparticle size of ≦20 μm, where M may be Fe, V, Cr, Co, Mo, Nb, Mn, Si,Sn, Al, B, Gd, Ge or combinations there of. M is added to in order tofurther reduce the braze temperature, substitute for the expensiveelements such as Zr or to improve desirable properties, such ascorrosion and oxidation protection by reducing for instance the Cucontent. A mixture of Ti, Cu, Ni and Fe powders 30-80 wt % Ti, 10-30 wt% Cu, 10-30 wt % Ni and 1-20 wt % M; placing the base material with thebraze material in a vacuum furnace; performing a first heating of thebraze material and the base material to achieve thermal stabilitybetween the braze material and the base material, the first heatingbeing up to a temperature that is not more than a first brazetemperature of the braze material; coating a second braze material ontothe base material, the second braze material being a mixture of Ti, Cu,Ni, M and Zr or Ag (PM) powders comprising 30-70 wt % Ti, 10-30 wt % Cu,10-30 wt % Ni, 1-20 wt % M, and 0.5-15% by weight Zr and or Ag (PM);performing a second heating of the braze material and the base materialup to a second braze temperature; and forming a braze joint between thebraze and the base material.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts the steps or acts of a brazing methodaccording to an embodiment of the present invention;

FIG. 2 is a side view of a base material brazed with a braze materialaccording to an embodiment of the present invention;

FIG. 3 schematically depicts the steps or acts of a brazing methodaccording to another embodiment of the present invention;

FIG. 4 depicts a scanning electron microstructure of a brazed joint ofTi21S made using a powder mixture of Ti Ni Cu Zr Ag with a particle size<44 μm (325 mesh). The uniformity of the braze and uniform distributionof the elements in the braze can be clearly seen.

FIGS. 5 a-c depict microstructures of a conventional braze; FIG. 5 a (TiNi Cu) carried out with a short exposure time, and of a braze of mixedpowder of <44 μm (<325 mesh); FIGS. 5 b and c also brazed for the sametime, but at a lower temperature. The extensive grain growth that occurswith the conventional braze and the benefit of reducing the brazetemperature can be clearly seen by comparing the grain size in FIGS. 5 aand 5 c.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention, since the scope of theinvention is best defined by the appended claims.

The present invention provides a braze material and method of brazingtitanium metals. The material consists of a mixture of Ti, Ni and Cupowders with a particle size less than 60 μm. Optionally, a secondbrazing may be performed to rebraze any braze joint that did not brazesuccessfully. The second brazing material may consist of a mixture ofTi, Ni, Cu plus Zr and or Ag (PM), also with a particle size less than60 μm. Because of this, minimal amounts of Zr and or Ag (PM) may beused, while still achieving higher percentages of brazes without leaks.Optionally a third brazing composition may be used comprising theTi—Ni—Cu, Ti—Ni—Cu—Ag, Ti—Ni—Cu—Zr or Ti—Ni—Cu—Ag—Zr systems alreadydescribed to which is added 1 to 20% of M with a particle size of ≦60μm, where M may be Fe, V, Cr, Co, Mo, Nb, Mn, Si, Sn, Al, B, Gd, Ge orcombinations there of. M is added to in order to further reduce thebraze temperature, substitute for the expensive elements such as Zr orto improve desirable properties, such as corrosion and oxidationprotection by reducing for instance the Cu content.

The braze materials described allow the braze temperature to be reducedfor the first and any subsequent brazes. This allows a significantreduction in the thermal exposure experienced by the titanium and braze.A temperature reduction is more beneficial than a reduction in the timeat temperature as described in the prior art.

FIGS. 1 and 2 depict the overall steps or acts of a brazing cycleaccording to one embodiment of the present invention. The base materials15 to be brazed together undergo a cleaning step or act 10. For oneembodiment, the base material 15 comprises Ti-15 Mo-3 Nb-3 Al 0.2 Si(Beta 21S). Either before, concurrently, or after the cleaning step 10,the components of a braze material are mixed in a mixing step or act 11to form a braze mixture. According to one embodiment of the invention,the braze mixture comprises 57 Ti-19 Cu-24 Ni. According to anotherembodiment, the braze mixture comprises 50 Ti-18Cu-22 Ni-10Ag. Accordingto another embodiment of the invention, the braze mixture comprises55Ti-20Cu-20Ni-5Fe. In a coating step or act 12, the braze mixture isapplied to at least one of the two pieces of base materials 15 to bejoined together. Next, the base materials 15 are heated in a heatingstep or act 13 up to a brazing temperature such that a braze joint 16 isformed between the braze materials and the base materials 15.Thereafter, the braze joint 16 is subjected to a cooling step or act 14,whereby the braze joint 16 solidifies.

In more particularly describing the steps or acts of an embodiment ofthe present invention, it should be understood that the base materials15 to be brazed can vary both in thickness and composition. However, thepresent invention can be extremely useful for thin foils or gages ofbase material 15 on the order of about 0.002 to 0.090 inches thick.Likewise, the brazing cycle is particularly useful for what may betermed “substantially isomorphous beta phase only titanium alloys.”Those alloys are intended to be distinguished from alloys that can betermed “substantially alpha phase only titanium alloys” and “alpha-betatitanium alloys.” Further, the term “substantially isomorphous betaphase only titanium alloy” is intended to mean an alloy that primarilyexists in a metastable beta phase at ambient temperatures. In contrast,the terms “substantially only alpha phase titanium alloy” and“substantially only near alpha phase titanium alloy” are intended tomean an alloy that primarily exists in an alpha phase at ambienttemperatures. Similarly, the term “alpha-beta titanium alloy” isintended to mean an alloy having substantial amounts of both alpha andbeta phases at ambient temperatures.

Examples of substantially isomorphous beta phase only titanium alloysinclude Ti-15 V-3 Cr-3 Sn-3 Al (Ti 15-3), Ti-15Mo-3Nb-3Al-0.2Si (Beta21S) and Ti-13 V-11Cr-3Al. Substantially alpha (or near alpha) phaseonly titanium alloys include commercially pure titanium (CPTi),Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-2.5Sn. The alpha-beta titanium alloysinclude Ti-3Al-2.5 V, Ti-6Al-4 V and Ti-7 Al-4 Mo.

Irrespective of the particular composition of the base material 15, thebase material 15 initially undergoes a cleaning step 10. The step or act10 is intended to remove oxides present on the surface of the basematerial 15, as well as to degrease the base material 15. The manner ofoxide removal and degreasing can occur by any known method practiced inthe art. For example, the base material 15 can first undergo acidpickling in about 35% volume nitric acid and about 3 oz/gallon ammoniumbiflouride for about one minute to remove surface oxides. Thereafter,the base material 15 can be degreased in isopropanol. Other suitablemeans for accomplishing the cleaning step 10 include pickling inaccordance with American Society of Testing and Materials (ASTM)specification B 600.

After the base material 15 is subjected to the cleaning step or act 10,the base material 15 can optionally be coated with a binder to assist inadhering the braze material to the base material 15 in step 12. Whilethe particular binder may vary, it is generally characterized as apolymer adhesive having a very low residual ash content and thatvolatizes completely at temperatures below about 1000° F. Some examplesof useful binders include Nicrobraz cements produced by Wall ColmonoyCorp. of Madison Heights, Mich. If the base material 15 is coated with abinder, the same preferably occurs just before the coating of the brazematerial. With such timing, the binder is still tacky and can betteradhere to the base material 15 and the braze material.

Either before, during or after the cleaning step 10, the brazecomponents of the braze material are mixed in the mixing step or act 11.As with the base material 15, the braze material can vary. But for anembodiment of the present invention, the braze mixture comprisessubstantially only of titanium, copper, and nickel. Also, optionallyzirconia (Zr), a precious metal (for instance Ag) or M where M may beFe, V, Cr, Co, Mo, Nb, Mn, Si, Sn, Al, B, Gd, Ge or combinations of themmay be added in reduced quantities. Further, it is preferred that theCu/Ni ratio be around 0.8 and that that portion of the mixture comprisesabout 45 wt. % and the Ti comprises about 55 wt. %. Usefully, therelative amounts of the constituents may be 40-70% by weight Ti inpowder form with a particle size less than 60 μm, 15-24% by weight Ni inpowder form with a particle size less than 60 μm, and 15-20% Cu inpowder form with a particle size less than 60 μm. According to anotherembodiment, the relative amounts may be 30-70% by weight Ti in powderform with a particle size less than 60 μm, 15-24% by weight Ni in powderform with a particle size less than 60 μm, and 1-25% Ag (PM) in powderform with a particle size less than 60 μm. The brazing material mayfurther comprise 0.5-20% by weight Zr in powder form and preferably 5%by weight with a particle size of 60 μm. A preferred embodiment of thebrazing powders has a particle size ≦20 μm.

A braze material can be in various forms. The forms have includedlaminated foils, amorphous or rapid solidification foils, and powders.The present invention may utilize a powder form, and specifically asmall particle size powder less than 60 μm and preferably about 20 μm.The pre alloyed powder form has been preferred because of itshomogeneity compared to laminated strip and its significantly lower costthan amorphous or rapidly solidified foil.

In forming the powdered braze material, the process can include wellknown means in the art. However, powders can be obtained by purchasingthem commercially, such as from Praxair in Danbury, Conn., or MicronMetals in Salt Lake City, Utah. In forming the braze powders, they canbe made for example by plasma rotating electrodes, gas atomization,reaction synthesis and mechanical comminution.

Plasma rotating electrode powders are made by melting a spinningelectrode using a plasma and the molten droplets (protected fromoxidation) are then collected in a catch basin. Gas atomization involvespouring molten alloy through a compressed gas stream that breaks up themolten alloy into droplets. The droplets solidify as they fall into acatch basin. Reaction synthesis powders are made by combining fineparticles (possibly elemental powders) to form larger particles of thedesired composition. Mechanical comminution employs the grinding orpulverizing of a pre-alloyed ingot. The ingot is made by taking knownweight amounts of the constituents and then melting them into an ingotshape. Mechanical comminution is sometimes preferred because of theeconomy of the process, but it presents safety concerns because of theexplosive nature of any fine powder and the tendency of the mechanicalcommination to produce sparks. These safety concerns are greatlyincreased with reactive materials such as Ti and Zr. Further explanationof several of these processes is provided in the American Society forMetals Handbooks by the American Society for Metal, Ninth edition(1984).

As described in the prior art section the mixing of the elementalpowders was an approach which was considered possible, but veryundesirable because of the non-homogeneity of the braze mixture andresultant braze joint, both in terms of composition and microstructure,which would result in a poor low strength joint. Also thenon-homogeneity of the elemental powders would result in a slow sluggishmelting of the braze, which would occur over a wide temperature range.

Upon completion of the mixing step 11, the coating step or act 12occurs. Therein, the braze mixture may be coated on one or both of thetwo base materials 15 to be brazed together. The amount of braze mixtureneeded to accomplish the brazing may vary according to the size of thejoint gap between the base materials. In general, the amount of thebraze mixture is that which is necessary to ensure good fusion of thesurfaces of the base materials 15. In the context of heat exchangers,typical joint gaps may range from about 0.001 to 0.003 inches. For thesesizes of joint gaps and others which might range from about 0.001 to0.01 inches, the amount of the braze mixture preferably used may beabout 0.1 to 0.5 grams/in². By so doing, adequate filling of the jointsmay be achieved. Upon one or both of the base materials 15 being coatedwith the braze mixture, the base materials 15 can be juxtaposed to oneanother and placed in a mating relationship, such as that shown in FIG.2 for purposes of example.

Next, the mated base materials 15 with the braze mixture therebetweenmay undergo the heating step or act 13. The heating may occur at a rateand up to a temperature to nearly melt the braze mixture. The heatingstep 13 may be carried out with the intent to provide a braze joint 16which is characterized by, among other things, uniformity of brazeconstituent concentrations across the width of the joint. It may also becharacterized by a substantially void free phase(s) and fillet formationwith a low contact angle, all of which may be determined upon opticaland scanning electron microscopy. As a consequence of the jointcharacteristics, the base material 15 may be characterized at thebase-joint interface by minimal grain erosion.

To achieve the desired joint characteristics or morphology mentionedabove, the heating step 13 preferably includes a step or act of securingthe mated base materials 15 together to prevent movement. Otherwise, theformation of the braze joint 16 can be disturbed with a resultingdeterioration of the above desired morphological characteristics.Various means can be employed to secure the base materials 15, such asby simply tying them with a nichrome wire or dead weight loading. Afterthe base materials 15 are secured, the base material with the brazematerial may be placed in a vacuum furnace, and the braze material andbase material heated for a given braze time to achieve thermalstability. The braze time may be less than 15 minutes and preferablyaround 5 minutes. The step of heating may be up to a temperature that isnot more than a brazing temperature of the braze material. The brazingtemperature may be between 900° C. and 950° C. and this may be at arange of 1.1 to 11° C./minute. This forms a braze joint between thebraze and the base material. The titanium metal may be an isomorphousbeta phase only titanium base material selected from the groupconsisting of Ti-15 V-3 Cr-3 Sn-3 Al and Ti-13 V-11 Cr-3 Al. The brazejoint may also be cooled and/or solidified.

At the brazing temperature, the temperature is held for a braze time ofabout 0 to 15 minutes and, more preferably about 5 minutes. The lengthof the brazing time is sufficient to allow wetting and fusion butavoiding erosion of the base material 15 or excess diffusion of thebraze mixture into the base material 15.

After the step 13 of heating, the base and braze materials are subjectedto the step or act 14 of cooling from the brazing temperature and downto room temperature. The cooling step 14 allows the braze joint 16 tosolidify. Various means of cooling the braze and base materials can beutilized, such as by vacuum and inert gas cooling. The rate of coolingcan be any rate that does not lead to excess distortion of the assembledbase materials 15. The term “excess distortion” is intended to mean lossof dimensions of the base material 15 details beyond a tolerable levelfor the desired application.

As shown in FIG. 3, the metals may also be heated twice, with a firstheating and a second heating. The purpose of a first and second heating,as compared to a single heating, may be to allow a rebraze of a partwhere the first braze was not 100% effective. The first heating act maybe generally intended to braze the material. However, where leaks occuror the brazing is not 100% effective a second brazing may be desirableto rebraze the parts.

A method of brazing a beta phase titanium metal is disclosed comprisingthe steps of coating a first braze material onto a base material. Thebase phase titanium metal may be an isomorphous beta phase only titaniumbase material selected from the group consisting of Ti-15 V-3 Cr-3 Sn-3Al and Ti-13 V-11 Cr-3 Al. The braze material may be a mixed 11 to forma mixture, as discussed previously, of Ti, Cu, and Ni powders. Accordingto another embodiment, the braze material may be Ti, Cu, Ni and Zr.According to another embodiment the braze may be Ti—Cu—Ni—Zr and or Ag(PM). According to yet another embodiment elements represented by M maybe added to these three combinations where M is Fe, V, Cr, Co, Mo, Nb,Mn, Si, Sn, Al, B, Gd, Ge or combinations of them. The uniform brazemicrostructure that is obtained from using mixed <44 μm powders ofTi—Ni—Cu—Zr—Ag is shown in FIG. 4. In addition to the SEM microstructureshown, EDX maps showing the distribution of the braze elements wereobtained (not shown), and they confirmed that simply mixing fine powdersdoes produce a uniform distribution of the braze elements as indicatedin the SEM image, despite the reports to the contrary in the prior art.The microstructure, FIG. 4, also indicates the braze has not been fullyoptimized and that too much braze material may have been used. Examplesof these brazes and the braze temperatures are shown in Table 1. Themixture 11 may then coat 12 a base material and be heated 13 by placingthe base material with the braze material in a vacuum furnace. A firstheating of the braze material and the base material may be performed fora first braze time to achieve thermal stability between the brazematerial and the base material. The first braze time may be between 3and 10 minutes and preferably 5 minutes. The first heating may be up toa temperature that is not more than a first brazing temperature of thebraze material. The first brazing temperature may be between 850° C. and950° C., preferably 900° C. and at a rate in the range of 1.1 to 11°C./minute.

The brazes may be checked to determine the success of brazing. Also, itmay be assumed that there are some brazes that need rebrazing withoutinspection. If a second braze is desired, a second braze material may bemixed 17. This may be a mixture of Ti, Cu, Ni, Zr, Ag (PM) M as used inthe first braze with further additions of Zr, Ag (PM) or M to furtherreduce the braze temperature, with possible compositions shown inTable 1. The second braze material may be coated 18 onto the basematerial. The base material may or may not be cleaned 21 prior to theapplication of the second braze material. The addition of the Zr, Ag(PM) or M provides a second braze with a melting point slightly lowerthan that of the first. A second heating 19 is performed of the brazematerial and the base material up to a second brazing temperature. Thesecond brazing temperature may be between 800° C. and 900° C.,preferably 830° C. and at a rate in a range of 1.1 to 7.7° C./minute. Asecond cooling 20 may also be performed.

TABLE 1 Ti Ni Cu PM Zr M Melt Temp. ID wt % wt % wt % wt % wt % wt % C.1 55 25 20 925 2 50 25 18 7 880 4 5 40 20 20 10 (Ag) 10 860 6 35 20 1515 (Ag) 15 840 7 50 22 18 10 (Ag) 880 8 45 25 20 10 (Pd) 900 9 45 25 2010 (Au) 900 10 45 25 20 10 (Pt) 900 11 53 25 20 2 (Si) 880 12 52 25 20 3(Co) 890 13 50 25 20 5 (Mn) 900 14 52 25 20 3 (Fe) 880 15 53 25 20 2(Cr) 900 16 53 25 20 2 (Y) 910 17 53 25 20 2 (Gd) 900 18 53 25 20 2 (Nb)900 19 53 25 20 2 (Mo) 900 20 53 25 20 2 (Sn) 890 21 52 25 20 3 (V) 89022 53 25 20 2 (Al) 890 23 52 25 20 3 (Ge) 890

Table 1 Illustrates the compositions of the braze mixture and theirbraze temperatures. The compositions are not optimized and are intendedprimarily to illustrate the possible alternatives. These possibilitiesare further illustrated by a braze comprised of a mixture of <60 micronspowder particles of 45 wt % Ti 25 wt % Ni 20 wt % Cu 5 wt % Al 2 wt % Sn3 wt % Si, this successfully brazed Ti foils at a braze temperature of860° F. This is not an optimized composition, but an example toillustrate the possibilities indicated in Table 1. The actual brazetemperature varies slightly with furnace and size of the parts to bebrazed. The above were all brazed using the same geometry parts in thesame furnace.

An objective is to reduce the cost of the braze and one way to achievethis is to use elemental powders that are readily available, rather thanprealloyed powders, which have to be specially cast in low volumes.However, certain of the additions described in Table 1 are readilyavailable in an alloy form, for instance AlSi prealloyed powders. Usingthese commercially available prealloyed powders may be in some casesless expensive than using the separate elemental powders.

An advantage of the braze compositions indicated in Table 1 is theability to reduce the braze temperature of both the first and secondbraze. This reduces the metallurgical coarsening of the base metals,such as grain growth. FIG. 5 shows how this can result in an order of 10or greater decrease in grain size of the finished part. The grain sizeof the conventional braze in FIG. 5 a is of the order of 1/10 of amillimeter while that of the add mixed powder FIG. 5 c is of the orderof microns. This reduction in braze temperatures also reduces the chanceof oxidation of the base Ti which due to the reactive nature of Ti isalways a problem even with good vacuum furnaces and the Ti tendency tooxidize increases rapidly with temperature. The reduced brazetemperatures are important advantages that result in increasedmechanical properties (strength, fatigue & ductility) of the finishedpart.

It should be understood, of course, that the foregoing relates topreferred embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

1. A brazing material for joining titanium metals comprising; a firstpowder for a first braze on a base material; a second powder for asecond braze on the first brazed base material: the first powdercomprising 54-76% by weight Ti, 12-24% by weight Ni, 12-22% by weightCu, and a precious metal (PM) wherein the (Cu+PM)/Ni ratio is between0.5 and 1.0; the second powder comprising Ti, Ni, Cu and PM; the secondpowder comprising 1-20 wt % more of PM than said first powder; andwherein the second powder has a brazing temperature lower than a brazingtemperature of the first powder.
 2. A brazing material as in claim 1,wherein said Ti, Ni, and Cu are in powder form with a particle size lessthan 60 microns.
 3. A brazing material for joining titanium metals as inclaim 1: wherein the second powder further comprises Zr; wherein thesecond powder comprises 0.5-12% by wt. Zr., 42-76 wt % Ti, 12-22% byweight Cu+PM; and wherein the (Cu+PM)/Ni ratio is between 0.5 and 1.0.4. A braze material as in claim 3, wherein said Ti, Ni, Cu, PM and Zrare in particle form with a particle size of less than 60 microns.
 5. Abrazing material for joining titanium metals comprising: a first powderfor a first braze on a base material; a second powder for a second brazeon the first brazed base material: the first powder comprising 30-80 wt% Ti, 10-30 wt % Ni, 10-30 wt % Cu, and 1-20 wt % M, wherein M isselected from the group consisting of Fe, V, Cr, Co, Mo, Nb, Mn, Si, Sn,Al, B, Gd, and Ge and any combinations thereof; the second powdercomprising Ti, Ni, Cu and M; the second powder comprising 1-20 wt % moreof M than said first powder; and wherein said Ti, Ni, Cu and M are inpowder form and have a particle size no greater than 20 microns.
 6. Abrazing material for joining titanium metals comprising: a first powderfor a first braze on a base material; a second powder for a second brazeon the first brazed base material: the first powder comprising 30-80 wt% Ti, 10-30 wt % Ni, 10- 30 wt % Cu+PM, and 1-20 wt % M, wherein the(Cu+PM)/Ni ratio is between 0.8 and 1.0, and M is selected from thegroup consisting of Fe, V, Cr, Co, Mo, Nb, Mn, Si, Sn, Al, B, Gd and Geor any combinations thereof; the second powder comprising Ti, Ni, Cu, Zrand PM; and the second powder comprising 1-20 wt % more of (PM+Zr) thansaid first powder.
 7. A brazing material as in claim 6, wherein said Ti,Ni, Cu, PM and M of the first powder are in powder form and have aparticle size less than 60 microns.
 8. A brazing material as in claim 6:wherein the second powder comprises 0.5-12 % by wt. (PM+Zr).
 9. A brazematerial as in claim 8 wherein said Ti, Ni, Cu, PM, M, and Zr of thesecond powder are in powder form and have a particle size less than 60microns.